1. Technical Field
The present invention relates to the printing or reproduction art and more particularly to the production of a member capable of acting as a direct negative or an offset master using resistive ribbon thermal transfer printing technology.
2. Description of the Prior Art
Resistive ribbon thermal transfer printing technology is well known in the art for providing high resolution, good quality printing, and is typically used in devices such as computer printers and typewriters. A common form of the technology essentially utilizes the combination of a source of current, such as a printing electrode with a set of styli, in combination with a printing ribbon having a layer of resistive material along with a layer of fusible ink. When current is applied by the electrode to the ribbon in selected areas according to a pattern to be printed localized resistive heating is produced in the selected areas of the resistive layer which causes melting of the adjacent areas in the fusible ink layer. The melted ink is then readily transferrable to a receiving medium, such as paper, to produce a printed pattern thereon. The resistive layer is commonly a carbon or graphite-filled polymer, such as a polycarbonate. A thin current return layer may also be provided as an intermediate layer in the ribbon consisting of a metal such as aluminum. Currents of approximately 20-30 milliamps are used to produce temperatures of about 100.degree. C. which typically melt the thermally fusible ink. Various embodiments involving resistive ribbon thermal transfer printing are described in detail in U.S. Pat. Nos. 3,744,611, 4,098,117, 4,400,100, 4,491,431 and 4,491,432 and an example of commercially available printers utilizing the technology are those sold by IBM Corporation under the name QUIETWRITER.TM..
A related technology that is also well known is electroerosion printing which is suitable for producing direct negatives and direct offset masters for use in lithography. Specific embodiments utilizing this technology may be found in U.S. Pat. Nos. 2,983,221, 3,786,518, 3,861,952, 4,399,758 and 4,086,853. In this technology the recording medium is typically a member or sheet composed of a support layer and a thin conductive layer. The support layer may be of paper, or a polyester such as MYLAR.TM., while the thin conductive layer is of a suitable metal such as aluminum. The pattern to be printed is produced using an electrode to remove portions of the thin aluminum layer by means of an electric arc. More particularly, a printing head which may be composed of multiple styli, typically of tungsten wire of diameters of 0.3-0.5 mil, is swept across the surface of the recording medium while maintaining good electrical contact between the tips of the styli and the conductive layer. Timed electrical pulses are applied to appropriate styli when passing over areas of the surface to be printed to produce arcing in a given pattern between the energized styli and the aluminum layer. The magnitude of the arcing is sufficient to cause local heating and removal of the aluminum in the given pattern by disintegration or vaporization.
To prevent scratching of the aluminum layer in areas where no arc is applied and to minimize head wear and fouling, the recording medium is normally provided with a base layer between the supporting substrate and the thin metal layer as well as an overlayer on the latter. Recording media of this type, depending upon the properties of the various layers, may be used to produce direct negatives and direct offset masters suitable for use in lithography. For example, to produce a direct negative, a transparent polymer support layer with a thin aluminum layer directly deposited thereon may be used as a recording medium and the electroerosion writing is used for producing a pattern in the aluminum layer. Since the substrate is transparent and the aluminum layer is reflective to light, the electroerosion writing will produce the required light transparent and light opaque regions in the medium needed to make a negative. Such a negative can be employed in a plate making machine to make a "master" by contact printing such as used in offset photolithography.
On the other hand, electroerosion printing can also be used to simplify the process for making masters or plates in conventional offset lithography. In such an application the recording medium is typically composed of: a support layer; a base layer, which is hydrophobic; an aluminum layer; and an optional overlayer. When the aluminum layer is electroeroded and the overlayer removed, regions of the alminum layer (unwritten areas) and of the base layer (written areas) will be exposed on the surface of the medium. As aluminum is hydrophilic, water will be attracted to the aluminum surfaces or unwritten areas while organic inks are repelled thereby. The written areas composed of the hydrophobic base layer surfaces will repel water but accept organic-based inks. The recording medium thus has become a direct master since the pattern of the information to be printed has been mapped onto the medium's surface in the form of regions having appropriate affinity to water and ink.
As noted, the fundamental recording medium is provided with a base layer and an overlayer to deal with the problems of the scratching of the aluminum layer and wear and fouling of the printing head. Typically, the base layer is a hard layer consisting of hard particles embedded in a suitable binder such as silica in a cross-linked cellulose binder. The overlayer is typically a lubricating, protective layer, comprised of a polymer including a solid lubricant, such as graphite in a cellulosic binder. However, if the problem of scratching were not present the fundamental recording medium composed of the substrate and aluminum layer combination could itself be used for direct master and direct negative applications. For example, a clear polymer sheet typically of polyesters could be used as the substrate since it would be transparent to light and hydrophobic, and aluminum as the metal layer would be opaque and hydrophilic, so that the resulting electroeroded medium would be suitable for use as either a direct negative or a direct master.
The present invention involves a system and technique which obviates the need for the base layer and overlayer and provides an improved recording medium suitable for use as a direct negative or direct master.